Various methods have been employed to control vibration in suspension systems. For example, primary control methods such as "Skyhook Control" described in U.S. Pat. No. 3,807,678 to Karnopp et al., "Relative Control" described in U.S. Pat. No. 4,821,849 to Miller, "Observer Control" described in U.S. Pat. No. 4,881,172 to Miller, "Continuously Variable Control" as described in U.S. Pat. No. 4,887,699 to Ivers et al., "Delayed Switching Control" described in U.S. Pat. No. 4,936,425 to Boone et al., "Displacement Control" as described in U.S. Pat. No. 5,276,623 to Wolfe, "Rate Control" as described in U.S. Pat. No. 5,652,704 to Catanzarite and Modified Rate Control" as described in U.S. Pat. No. 5,712,783 to Catanzarite are used to determine the appropriate primary control signal to an electro-mechanical device, such as a controllable damper based upon various sensor inputs.
Under certain conditions, some or all of these primary control methods will result in abrupt collisions with the end stops (hereinafter referred to as "end stop collisions"). An end stop collision is where the mechanical system in which the damper is connected hits the end stops, i.e., the maximum mechanical limits of the extension and/or rebound strokes when a sufficient transient load is encountered. Generally, there are hard stops that are designed into the system, such that the damper itself does not see the brunt of the shock. However, if the system velocity is high enough when the end stop collision occurs, a very disconcerting rapid impact can occur. Of course, this bottoming and topping out imparts unwanted stresses to the mechanical components in the system (ex. linkages, swing arms, bushings, joints, etc.) and is detrimental to the system's overall life. Moreover, such collisions can be an annoyance to the driver.
By way of more explanation, the suspension system being controlled generally includes an electro-mechanical device (ex. a controllable orifice damper, a magnetorheological damper or an electrorheological damper, etc.) which is attached between two relatively moveable members. The device's damping is controlled to minimize vibration, but also to avoid end stop collisions. For example, in a controllable damper suspension system, a variable damper is attached between two relatively moveable system components, such as a vehicle chassis and suspension or, alternatively, between a vehicle seat and a structural body. One or more sensors provide information regarding the movement of the components of system, ex. relative or absolute displacement, velocity or acceleration. The damping characteristics of the damper are then controlled in accordance with any of the afore-mentioned primary control methods. The control may also include an overriding end stop control method.
An end stop control method is a method which operates in conjunction with, or in the background of, the primary ride control method (such as those mentioned above) to override or modify the primary control instructions should it be determined that an end stop collision is imminent or likely. Generally, the signal from the primary control and the signal generated from the end stop control are additive. One very effective end stop method which has been employed is described in U.S. Pat. No. 5,276,622 to Miller et al. entitled "System for Reducing Suspension End-Stop Collisions". The overall control signal V.sub.overall, which includes contributions from the primary control method and end stop control method, provides a digital signal value which is generally converted to an output voltage or current. This control signal to the damper includes primary control inputs V.sub.primary and end stop control inputs V.sub.end stop. This overall control signal V.sub.overall provided to the damper is represented by: EQU V.sub.overall =V.sub.primary +V.sub.end stop
where: EQU V.sub.overall =the overall command signal to the damper, EQU V.sub.primary =the portion of the signal due to the primary control method, and EQU V.sub.end stop =the portion of the signal due to the end stop control method.
It should be recognized that if it is determined by the end stop control method that an end stop collision is unlikely, then V.sub.end stop is set to equal zero. Thus, under this scenario, the overall damper control signal V.sub.overall is dictated by the primary control method only.
In a preferred implementation described in the Miller et al. '622 patent, the end stop control method calculates a maximum allowable relative velocity Vrm based upon available data and/or inputs such as relative velocity and relative displacement (see col. 10 of the '622 Miller patent). In the simplest form of the preferred implementation of Miller, the method comprises three steps.
Step 1: Determine the positive distance to the appropriate end-stop limit based on the direction of motion. The positive distance is given by EQU .delta..sub.end =.delta..sub.max -.delta..sub.inst when V.sub.inst &gt;0 EQU .delta..sub.end =.delta..sub.inst -.delta..sub.min when V.sub.inst &lt;0 PA0 Step 2: Determine an "error" value (e) according to the relation EQU e=Abs(.alpha.V.sub.inst)-sqrt(.delta..sub.end) PA0 Step 3: Determine the end stop control signal V.sub.end stop which is preferably added to the primary control signal V.sub.primary for damper-like output devices. ##EQU1## where EQU .beta.=a tuning constant. PA0 (a) determining an instantaneous relative velocity (V.sub.inst) based upon a sensor output from at least one sensor, PA0 (b) calculating an error value based at least in part upon a square of the instantaneous relative velocity (V.sub.inst), PA0 (c) calculating an end stop control signal (V.sub.end stop) based upon the error value (e), if the error value (e) is positive, PA0 (d) providing the end stop control signal (V.sub.end stop) to an output device, and PA0 (e) repeating steps (a) through (d). PA0 (a) determining an instantaneous relative displacement (.delta..sub.inst) based upon a sensor output from at least one sensor, PA0 (b) setting a snubbing zone (Z) adjacent to an end stop limit, PA0 (c) determining whether the instantaneous relative displacement (.delta..sub.inst) is within the snubbing zone (Z), PA0 (d) determining a snubber intrusion distance (.delta..sub.z) which represents an distance of intrusion into the snubber zone (Z) from a point of first entry into the zone (Z), PA0 (e) calculating a snubber control signal (V.sub.snub) based at least in part upon a continuous function of the snubber intrusion distance (.delta..sub.z), PA0 (f) providing the snubber control signal (V.sub.snub) to an output device, and PA0 (g) continuously repeating steps (a) and (c) through (f). PA0 (a) determining an instantaneous relative displacement (.delta..sub.inst), PA0 (b) determining an instantaneous relative velocity (V.sub.inst), PA0 (c) calculating an error value (e) based at least in part upon a square of the instantaneous relative velocity (V.sub.inst), PA0 (d) calculating an end stop value (V.sub.end stop) based upon the error value (e), if the error value (e) is positive, PA0 (e) setting a snubbing zone (Z) adjacent to an end stop limit, PA0 (f) determining whether the instantaneous relative displacement (.delta..sub.inst) is within the snubbing zone (Z), PA0 (g) determining a snubber instrusion distance (.delta..sub.z) which represents an distance of intrusion into the snubber zone (Z) from a point of first entry into the zone (Z), PA0 (h) calculating a snubber control signal (V.sub.snub) based at least in part upon a continuous function of the snubber instrusion distance (.delta..sub.z), PA0 (i) providing the snubber control signal (V.sub.snub) and the end stop control signal (V.sub.end stop) to an output device, and PA0 (j) continuously repeating steps (a) through (d) and (f) through (i).
where EQU .alpha.=a tuning constant, EQU V.sub.inst =the instantaneous relative velocity, and EQU .delta..sub.end =the distance to the nearest end stop limit.
The Miller '622 end stop method includes two signal inputs: 1) the relative displacement .delta..sub.inst, and 2) the relative velocity V.sub.inst. Using an output signal from a position sensor whose output is indicative of the relative displacement .delta..sub.inst, a relative velocity V.sub.inst. estimate is obtained from passing the signal through a well-known "differentiation" filter, such as a "Rate" filter or by taking a simple "Euler derivative".
The Miller '622 end stop method also requires the specification of two positive constant parameters: .alpha. and .beta.. The .beta. parameter is simply a gain which increases or decreases the amount of end stop control present. The constant .alpha. determines the parabolic shape of the control surface. Both parameters are tuning parameters which are set based upon trial and error. As best shown in FIG. 3a, it can be readily seen that the shape of the end-stop control surface causes system trajectories to be re-directed away from most end-stop collisions, i.e., an increase in the end stop control signal causes an increase in the current which concomitantly increases damping applied. The increased damping, in turn, causes a decrease in the magnitude of the relative velocity V.sub.inst.
For example, referring to FIG. 3a, if the displacement were about 0.75 inch and the velocity were about 40 in/sec, as indicated by point 37 on the control surface 38, the end stop control would accordingly deliver the current I in amps such that the damping is increased. This, of course, will attempt to avoid a hard end stop collision. Moreover, it should be recognized that when operating within the confines of the flat bottom 40 of the control surface 38, no end stop control signal is commanded. However, there still may be current to the damper from the primary control method. See for example, FIG. 3b which illustrates the control surface 38a for the previously mentioned "Rate Control" method as described in U.S. Pat. No. 5,652,704 to Catanzarite.
It should also be recognized that in the preferred implementation of the Miller '622 patent, a computation of a square root function is required to determine Vrm. Vrm is the velocity above which an end stop collision is imminent (see FIG. 4, block 210 of Miller '622). Determining a square root calculation in either a fixed point or floating point processor is very computationally expensive thereby requiring an expensive microprocessor. Further, square root calculations require significant memory resources, thus adding unwanted cost and complexity to the system.
Moreover, for certain conditions, such as where relative velocities are low, it is possible to have an end stop collision even though the overriding end stop method is present and operational. For example, when a user gets off of an air spring suspended seat including a controllable damper suspension, such as is described in U.S. Pat. No. 5,652,704 to Catanzarite entitled "Controllable Seat Damper System And Control Method Therefor", the seat tends to be driven to the top of its travel limit at a low, yet still significant velocity. This is an example of what will be referred to herein as a low-velocity, high-displacement condition. This causes an unwanted jarring impact to the seat system components. Notably, this is because the preferred implementation of the '622 Miller end stop control patent is incapable of preventing collisions in the area of the control surface. approximately designated as 42 (FIG. 3a). This is because there is only a small ramp up in current which is too little and too late to avoid a collision.
Therefore, there is a long felt, and unmet, need for a simple and cost effective method(s) for further avoiding end stop collisions in controllable systems.